1. Field
The present invention relates generally to communications. More particularly, in aspects the invention relates to receiver automatic gain control acquisition in time division duplex wireless communication systems.
2. Background
Modem wireless communication systems are widely deployed to provide various types of communication applications, including voice and data applications. These systems may be multiple access systems capable of supporting communications with multiple users by sharing the available system resources (e.g., spectrum and transmit power). Examples of multiple access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, time division duplexing (TDD) systems, frequency division duplexing (FDD) systems, 3rd generation partnership project long term evolution (3GPP LTE) systems, and orthogonal frequency division multiple access (OFDMA) systems. There are also point-to-point systems, peer-to-peer systems, wireless local area networks (wireless LANs), and other systems.
Generally, a wireless multiple access communication system can simultaneously support communications with multiple wireless terminals. Each terminal communicates with one or more base transceiver stations (a k a BTSs, base stations, access points, Node Bs) via transmissions on the forward and reverse links. The forward link (FL) or downlink (DL) refers to the communication link from a base transceiver station to a terminal, and the reverse link or uplink refers to the communication link from a terminal to a base transceiver station. Both forward and reverse communication links may be established via single-in-single-out (SISO), multiple-in-single-out (MISO), single-in-multiple-out (SIMO), or multiple-in-multiple-out (MIMO) communication techniques, depending on the number of transmitting antennas and receiving antennas used for the particular link.
MIMO systems are of particular interest because of their relatively higher data rates, relatively longer coverage range, and relatively more reliable data transmission. A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels (also referred to as spatial channels), where NS≦min{NT, NR}. Each of the NS independent channels corresponds to a dimension. MIMO systems can provide improved performance (e.g., higher throughput and/or improved robustness) if the additional dimensions created by the multiple transmit and receive antennas are used.
MIMO techniques may support both TDD and FDD systems. In a TDD system, the forward and reverse link transmissions are in the same frequency band. In an FDD system, different frequency bands may be used for forward and reverse link transmissions.
In many wireless communication systems, a receiving device, such as a mobile device or user equipment (UE), determines estimates of the forward link physical channel between itself and a base station. The forward link physical channel estimates may be used to demodulate the signal the UE receives from the base station, for adaptive power control, and/or for other functions.
In UEs, downlink channel estimates are typically obtained from pilot signals or simply pilots. A pilot is a reference signal known a priori to a particular UE (and possibly to all UEs or a subset of all UEs). In addition to the pilots used for channel estimation and demodulation, there may be synchronization signals that allow the UEs to acquire system timing on power up. Such signals, known as Primary and Secondary Synchronization Signals (PSS and SSS, respectively) are also known a priori.
The dynamic range of received signals at a UE's antenna may be quite large. Typically, a UE needs to accommodate about 100 dB of signal power variation. At the same time, the dynamic range determined by the bit-width of the UE's analog-to-digital converter (ADC) is typically about 90 dB. Additionally, about 65 dB of the ADC's dynamic range may be needed to provide operating margin and to accommodate various effects, such as carrier-to-interference ratio, channel variation due to Raleigh fading, shadowing, I/Q ratio, peak-to-average ratio, and quantization noise. To accommodate the difference between the needed dynamic range and the dynamic range available at the ADC, several AGC states may be used to vary the analog radio frequency (RF) gain in the front end of the UE's receiver. The analog RF gain may be (or may include) the gain of a low noise amplifier (LNA). Each AGC state may correspond to a predetermined analog RF gain; the states may increase with an increment of a predetermined size, for example, 10 or 15 dB.
Proper DL reception requires the AGC to be in an appropriate state (or one of a few appropriate states), so that the signal level at the ADC input falls within an acceptable range. Once the DL signal is acquired, the AGC state may be adjusted in a closed loop (feedback-controlled) manner. In the case of FDD, where only DL frames are transmitted on a given DL frequency, the AGC state may also be acquired in a closed loop manner, even before network timing is known.
In the case of TDD, however, AGC acquisition is more complicated, because both DL and UL subframes of 1 ms may be present within a single 10 ms frame. In TDD, the AGC acquisition problem is more difficult to deal with, because the UE does not have the information regarding boundaries of subframes or which subframes are DL subframes, and because the dynamic range of even adjacent subframes can exceed 100 dB. For cell edge operation, for example, the DL signal may be quite weak, while another UE can be located nearby and transmit at full power on the UL—at the same frequency and within the same frame. In essence, the UL subframes may interfere with AGC acquisition.
One way solve this problem is for a receiver (e.g., UE) to use a given gain state (e.g., “1”), to try to acquire the system. If the acquisition attempt fails in the selected gain state, then the receiver may repeat sequentially for other gain states (e.g., gain state 2, gain state 3, and so on), until the system is successful acquired. This brute-force approach may take a long time, particularly if the appropriate AGC state is the last state in the sequence of states that the AGC is programmed to try sequentially.
A need thus exists in the art for apparatus, methods, and articles of manufacture that reduce the time for AGC acquisition in TDD systems. A further need exists for apparatus, methods, and articles of manufacture that reduce expenditure of UE power and computational resources during AGC acquisition in TDD systems.